This relates to the field of Orthopedics and Trauma, human or veterinary. It can be used for other biological fixations/immobilizations such as botanical or other forms of life and for tissues other than bone. It can be used in any engineering or mechanical endeavour in which it serves to hold and/or compress together fragments or masses of material together, while taking part in an outside construct at a distance from the fragments.
Bone is living tissue. Bone fragments and surfaces can unite by biological activity over a length of time, given proper conditions to favor it. During this biological process of healing, the fragments have to be held together continuously by various means, to achieve a finally acceptable result for restoring function to the part.
The biological process is favoured by the following measures.                1. Immobilization of the fragments or surfaces attempting union.        2. Compression of the surfaces to increase the rigidity of immobilization, and also promoting the biological process of direct union without excessive callus formation.        3. Relieving recurrent stress and injury to the soft tissues and neuro-circulatory mechanisms by immobilization.        4. Immobilizing only the healing parts, while encouraging movement and activity of un-injured parts.        This has been attempted by the following methods.        A. Continuous traction        B. External casts of Plaster of Paris, other casting materials and bracing.        C. Internal fixation.        D. External fixation.        E. Combined methods of fixation.        A. Continuous Traction:        
This can restore the length of the limb, and further measures can correct rotation and angulation to an extent.
The following problems of this method seldom make it the preferred treatment.
1. It is difficult to maintain the traction force continuously even with very frequent attention.
2. Patient cooperation is difficult to achieve.
3. Due to intermittent loss of traction force, malunion may occur. Distraction and movement of fragments may cause delay or failure of union.
4. Circulatory problems can occur in the distal limb.
5. Wounds in the traction surface will not allow such treatment.
                B. External Casts of Plaster of Paris, Other Casting Materials and Braces.        
The following problems are associated with them.
1. The immobilization is not rigid enough, when this is critically essential.
2. Encircling of the part causes sweating and discomfort in hot climates.
3. Pressure sores can occur at pressure points, or due to insertion of hard objects by patient for scratching. Bugs can get in.
                5. Swelling of part within the cast can cause tightness and loss of circulation or nerve function.5. Loosening of cast occurs due to loss of swelling of the part, or due to reduction of the thickness of the padding by moisture, resulting in loss of reduction.        6. There is no access to any wounds inside, which may need regular attention, except by cutting out windows or leaving the cast incomplete, which may jeopardize the immobilization, and fracture position.7. Uninvolved parts also are immobilized, a setback to recovery.        
Due to these factors it can suffice only when rigid immobilization is not critically important, and usually in the absence of complicating factors of wounds and circulation.                C. Internal Fixation:        
This may be applied along the side of a bone in the form of a plate and screws of any preferred design. It allows accurate reduction when this is most desirable; a bone graft can be added and lag screws driven as often as feasible, for inter-fragmentary compression. Sliding devices can be added to passively close any gaps arising later. Disadvantages are as under:                a. Large exposures are required with relatively greater damage to the soft tissues and bone circulation. Meticulous technique may minimise this, yet the exposure is larger.        b. Compression between fragments once applied at operation wears off within hours, depending on the quality of bone. There is no possibility of renewing this compression once the wound is closed over the device. It is not acceptable to re-anaesthetise and re-expose the device repeatedly to re-tighten the screws.        c. Minimally invasive methods are performed through smaller incisions but in order to place the plate directly on bone, the periosteum and muscle have to be stripped blindly. The plate is always unavoidably placed over some soft tissues, which melt away under the pressure and loosen the plate. Loss of torque of screws is unfavourable to biology of bone healing.        d. Plates are seldom favoured in compound fractures.        e. Fracture haematoma gets dispersed.        
Internal fixation may be applied inside the medullary canal of bone in the form of nails, pins and wires.
In closed nailing, the fracture haematoma is preserved.
The disadvantages are as under.                1. It is generally not applicable to children, due to growth plates at the ends of bones.        2. It invades and occupies the bone from end to end, with the possibility of spreading infection.        3. It is not stable to rotational forces, and interlocking methods are not available for all situations.        4. In open nailing, the fracture haematoma is dispersed.        D. External Fixation:        
This is most suited for open injuries of bone. The commonly used basic bone implant for the external fixator is the Schanz screw which can be inserted at a safe distance from the open wounds and fracture ends.                1. Access to wounds for frequent attention is easy.        2. There is no aggravation of injury to bone or soft tissue.        3. Safe corridor entries of screws prevent injury to neuro-vascular structures.        4. In transverse fracture patterns, some compression can be applied along the axis of the bone by dynamizing the construct.        
The following limitations remain:                1. The basic implant e.g. the Schanz screw has a tendency to loosen in bone, leading to instability and a proneness to infection. Radial preloading of the implant in bone improves the stability, by the technique of inserting a larger diameter screw in a suitably smaller diameter drill hole, but the rod/bone interface remains small.        2. The preload is only in one mode, viz. Radial, over a small area.        3. After loosening, there is no way of regaining any degree of stability in the same position, before the onset of infection. If the loose screw had been initially placed in a mechanically ideal site, then any next site for repositioning will be less than ideal.        4. There is no lag screw effect of a Schanz screw, to exert inter fragmentary compression. Inter-fragmentary compression greatly enhances the stability, as well as the biological process of union. Fragments can at most be splinted across, but not drawn together and compressed in the lag screw mode, by the conventional Schanz screw.        5. Taylor et al had patented a lag screw external fixator implant with intercalated heads of various shapes, meant for compression of fragments. All the heads patented by them had engagement surfaces which were flat, their plane being at 90-degrees to the rod axis. Unless such a device is always driven at 90-degrees to the bone surface, the head will stand up on edge, with too much stress concentration at a point, leading to crumbling of bone and loosening.        
Also, two fracture surfaces are best compressed by a lag screw driven at 90-degrees to the fracture plane and not at 90-degrees to the outer surface.
Since fracture planes run across a bone, they are at various angles to the outer surface of bone and not parallel to it.
In actual use, the devices of Taylor et al would not be at 90-degrees to the outer bone surface, being required to be at 90-degrees to the fracture plane. This would make the engagement surface of head stand up at an angle to the surface leading to mechanical failure and loosening. This may be the reason, why the device did not gain wide acceptance amongst those skilled in the art, and for the paucity of any reports on its use in English literature.
If compression were applied at 90-degrees to outer bone surface, but not at 90-degrees to fracture plane, it will not only be inefficient but may actually make the fracture surfaces to slide upon each-other rather than be compressed.
Taylor et al suggested that their device might also be used as “traditional external fixator” screw, in which case the head may be allowed to stand away from the bone surface, without engaging or loading the bone surface.
For such use as a “traditional” device, it must be considered that an intercalated head adds to the cost of the implant and needs a little larger entry incision. Unless the head is going to do some work, the surgeon skilled in the art will not use it, preferring a conventional implant widely available, rather than pay for an intercalated head and not put it to use. This could have ruled out its acceptance even as a conventional implant.
Taylor et al thus did not provide for the need of inserting a lag screw device at different angles to the outer surface of bone, which is the crux of the art of lag screw fixation.
Taylor et al do not mention a spherical shape in their claims or any part of text, nor does their discoid head with rounded edges stand up to a close geometric scrutiny for being “spherical on account of being a segment of a sphere”. In a similar way, none of the heads claimed by Taylor et al satisfy the criteria of being “conical”, when scrutinized geometrically. They describe a “Gemini capsule” shaped head at their FIG. 6. This head they refer to as being “conical” on one occasion at column 7, lines 32-34 of their application. This Gemini capsule head is in fact quite pear-shaped, and unlike any geometrical cone.
Ignoring Botanical interpretations of the term Cone, a Cone has been defined at Merriam Webster Online Dictionary as:
“A solid generated by rotating a right triangle about one of its legs—called also a right circular cone.” Or “A surface traced by a moving straight line passing through a fixed vertex.” Or, “A solid bounded by a circular or other closed plane base and the surface formed by line segments.”
“The “Gemini capsule” shape head seen at FIG. 6 of their patent has a narrower base which soon expands to a low belly in a curved line and then tapers to the vertex.
Geometrical cones are widest at the base, tapering in straight lines to the vertex in ever reducing cross-sectional areas, without any widening from base upward. If curved lines are allowed in the geometry of a cone as in the Gemini capsule, with expanding and reducing cross-sections, then bizarre shapes can compete for the definition of a cone. The preferred conical head therefore as claimed in this application for a basic implant, is not anticipated by any device of Taylor et al, who claim only a lag screw device and not a basic implant.                E. Combined Methods of Fixation:        
When any one method is inadequate to neutralize all the forces of muscular pull and gravity, another method is added onto the first. For example, in “mini-cum-external fixation” methods; one or two lag screws used to hold together some fragments, are supplemented by an external fixator construct, or by traction.
Even with such a supplementation, the lag screws can fail, because by the blind stab-hole technique of insertion, there is always some interposition of soft tissue between the screw head and the bone surface. This soft tissue quickly undergoes pressure necrosis to loosen the compression by loss of torque. The only residual control is the external fixator, which may not be adequate for the situation. The compression once lost cannot be regained.